Generally, during ink jet printing, ink is ejected onto a substrate in controlled patterns of closely spaced dots to produce printed text, graphics, holograms, and other images.
Both solvent-based and radiation-curable inks are currently available for ink jet printing. Inkjet printing with aqueous-based or organic solvent-based formulations generally includes a drying operation during which the water or organic solvent present in a deposited fluid ink dot is evaporated, leaving a solid print residue on the substrate. While organic solvents generally have high vapor pressures and are easily evaporated during drying, they pose environmental, safety or health hazards and often require special handling. Water-based inks are safer to use; however, drying such inks generally is more energy-intensive.
Radiation curable inks are hardened by exposure to radiation. To address drying-related problems described above, these inks generally are formulated to minimize and preferably eliminate the use of non-reactive diluents. For example, several existing radiation curable inks have been formulated to rely on “reactive diluents,” such as functionalized monomers that, together with other ingredients, crosslink to form the printed image.
Radiation curable inks are printed onto numerous substrates, both rigid and flexible. These include polymeric substrates, such as various types of PVC as well as polystyrene (usually modified), polycarbonate, acrylonitrile-butadiene-styrene (ABS), polyolefines, polyesters, and others. The appearance of the printed image is affected by “jettable” characteristics of the ink, such as ink viscosity, droplet formation, satellite formation, drying or curing time and other properties that relate to the ejection of the ink from the print head, droplet travel and impact onto substrate.
When used for outdoor displays, such as billboards, busboards, or vehicle decoration, many of the existing ink formulations are found to result in premature peeling or flaking. Some exhibit poor flexibility when folded or wrinkled as evidenced by cracking and/or crazing. In many cases, existing ink formulations perform well on a specific substrate material but are found to have poor adhesion when printed on a different substrate. On the pencil hardness scale, existing formulations often are soft, typically having a hardness that is less than B.
Therefore, a need continues to exist for radiation-curable ink compositions that adhere well to multiple substrates, form printed images having good appearance, improved hardness and good flexibility and that are capable of withstanding outdoor weather conditions.